Circuit breakers provide automatic current interruption to a monitored circuit when undesired fault conditions occur. These fault conditions include, for example, arc faults, overloads, ground faults, and short-circuits. Referring to FIG. 1, a portion of an exemplary prior art circuit breaker 1 is shown. In the circuit breaker 1, an overcurrent is detected when the fault current generates sufficient heat in a terminal 20 and a bimetal 10 to cause the bimetal 10 to deflect and/or bend. The mechanical deflection triggers a trip assembly that includes a spring-biased trip lever 50 to force a moveable contact attached to a moveable conductive blade away from a stationary contact, thereby breaking the circuit. When the circuit is exposed to a current above that level for a predetermined period of time, the trip assembly activates and tripping occurs thereby opening the circuit.
The bimetal 10 deflects in a predictable and repeatable manner across a thermal profile over a period of time. The bimetal 10 is attached to a yoke 30 that is magnetically coupled to a moveable armature 40. The movement of the bimetal 10 in response to excessive electrical current causes the yoke 30 to move the armature 40, which triggers a chain of mechanical actions that cause the circuit breaker 1 to thermally trip. For magnetic tripping in response to sudden overloads (e.g., a short circuit condition), a magnetic field induced relative to the magnetic yoke 30 causes the armature 40 to be moved relative to the yoke 30, which triggers a chain of mechanical actions that cause the circuit breaker 1 to magnetically trip. However, the circuit breaker 1 is unable to magnetically trip in certain situations, such as extreme cold environments without a thermal-assist. Such a situation is illustrated in FIG. 1 as the armature 40 is pulled to an inner surface of the yoke 30 yet the trip lever 50 remains engaged with the armature 40.
The circuit breaker 1 also includes a solenoid 70 coupled to electronic components that detect one or more fault conditions and are operable to cause the circuit breaker 1 to electronically trip. The solenoid 70 and the electronic components can be in addition to or in lieu of the thermal-magnetic tripping components. The electronic components process a signal output of a sensor that monitors current flowing in the circuit breaker 1. The electronic components are configured to determine whether one of the fault conditions is present and to generate a fault signal and/or a trip signal. In response to the generation of a fault signal, a magnetic field is created around the solenoid 70, causing a plunger to move the armature 40 relative to the yoke 30, which triggers a chain of mechanical actions that cause the circuit breaker 1 to electronically trip.
Extreme cold temperature conditions, for example, negative thirty-five degrees Celsius, may cause the bimetal 10 to deflect in the direction of arrow B with a “bimetal deflection force,” which requires the circuit breaker to use either thermal-assist or a solenoid to overcome the bimetal deflection force to trip. For a magnetic trip, a magnetic deflection force can cause the bimetal 10 to take a mechanical set that increases the tripping time beyond allowable limits. Similarly, for an electronic trip, the additional bimetal deflection force, which can be multiple times as great as normal latch engagement forces, requires an auxiliary trip device, such as the solenoid 70, to reliably overcome the latch engagement forces between the trip lever 50 and the armature 40 and the bimetal deflection force. Thus, the circuit breaker 1 employing an auxiliary trip device must include a larger solenoid 70 to trip the circuit breaker in cold environments within the allowable tripping time. These larger solenoids are physically larger that generate a larger pull force, however, the larger solenoids also require larger overall circuit breaker housings. Thus, in practice, as space in a circuit breaker is at a premium, miniature circuit breakers tend to not supply large solenoids and therefore may not operate effectively in extreme ambient temperatures, such as, for example, negative thirty-five degrees Celsius.
In miniature circuit breakers, such as the QO® and HOMELINE® family of circuit breakers available from Square D Company, the outer dimensions of the circuit breaker housing limit the size of the solenoid that can be used. In these circuit breakers having such solenoids, cold performance may not be achievable.
Thus, a need exists for an improved apparatus and method. The present invention is directed to satisfying one or more of these needs and solving other problems.